Unit 7: Nuclear Chemistry Colin Johnson and George Fourkas

Nuclear Decay Decay Particles Alpha Beta Positron Emission Electron Capture

The element loses two protons and two neutrons, decreasing the mass number by four and atomic number by two for each alpha decay. Alpha decay is the most common in elements with an atomic number greater than 84. An alpha particle is also known as a He-4 particle ALPHA DECAY

Beta Decay One neutron changes into one proton and one negative beta particle. The atomic number (Z) increases by one due to the new proton. The mass number (X) is unchanged because a neutron is gone. A negative Beta particle is also formed to keep the particle neutral. Beta decay follows the form:

Electron Capture The nucleus captures an electron from the electron cloud. The electron is consumed rather than formed, so it is included on the reactant side of the equation.

POSITRON EMISSION One proton changes into one neutron and one positron particle. The atomic number decreases by one due to the loss of a proton. Since it changed into a neutron, the mass number is unchanged.

Zone of Stability The stable nuclei are in the shaded area (belt of stability). Radioactive nuclei are outside and must go through some type of decay to regain stability. N/P –if N is too high, usually beta decay occurs. –If N is too low, usually positron emission or electron capture occurs. –For nuclei with atomic numbers over 84, alpha decay occurs.

Sample Decays U  He + Th Pb  Bi + β Mn  Cr + e W + e  Ta Alpha Decay Beta Decay Positron Emission Electron Capture

Nuclear Energy Binding Energy: The energy required to separate a nucleus into its individual nucleons E=Δmc 2 –E the binding energy –m the mass defect –c 2 the speed of light ( x 10 8 m/s)

Nuclear Energy Mass defect: the mass lost when a nuclear change occurs. – the mass of the products is less than the mass of the reactants –Example: Calculate the binding energy per nucleon for C-12 C= amu Proton= amu Neutron= amu 12 6

Half Life (t 1/2 ) Radioactive elements decay according to their half life. The time required for the concentration of a reactant substance to decrease to half its initial value; the time required for half of a sample of a particular radioisotope to decay.

Rate of Decay The decay rate is the speed at which a substance disintegrates. The following equation represents the relationship between the number of nuclei remaining, N, the number of nuclei initially present, NO, the rate of decay, k, and the amount of time, t. N=N o e -kt K=ln2/t 1/2

N=N 0 e -kt Activity = -dN dt = kN S T0T0 -dN N S T0T0 = dtdt k ln N t -ln N 0 =-k(t-0) Nt=N0e-kt